Thread: "16-bit" mode.
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Old November 16th 04, 05:28 AM
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Default "16-bit" mode.

First I thought it was strange when I found out that Photoshop's 16-bit
mode was actually 15-bit. Then, I though it was stranger still when I
found out there was one additional level (32768; it was explained that
this made certain types of math faster). Now, this evening, I was
writing an applet to extract RAW data from the uncompressed output of
the DNG converter, and decided to create a raw greyscale bitmap 256*256,
containing every possible 16-bit value, to see what PS would do with
them. I assumed that PS would make 0 and 1 0, 2 and 3 1, 4 and 5 2,
etc, but it does not. This is a sample of what it does:

real
16-bit PS values
data

0 0
1 0
2 0
3 3
4 3
5 3
6 3
7 3
8 3
9 5

2044 19128
2045 19131
2046 19131
2047 19131
2048 19131
2049 19131
2050 19131
2051 19134
2052 19134

4085 32762
4086 32762
4087 32766
4088 32766
4089 32766
4090 32766
4091 32766
4092 32766
4093 32768
4094 32768
4095 32768

Does anyone have any idea why they posterize the data even more than the
15-bit limitation?

This is basically about 13.5 bits of level-resolution.
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John P Sheehy